Inverter and distorter for secret communications



ug- 316 1949- L.. E. GABmLovl-rcH ZAQSS INVERTER AND DISTORTER FOR SECRET COMMUNICATIONS Filed Jan. 15, 1945 2-Sheeos-Sheet l I HF. MODULATOR ELECTRONIC SWITCH DOUBLE BAL MoDuLAToR PHASE sH l FTER LQP. FILTER BALANCED MODULATOR SELECTOR SELECTOR 13.?. FILTER CLIPPER OSCILLATOR .Z50/m71 El @ABR/LoL//Tcf/ INVENTOR.

BY /MMm/ww ug. i6, E949. L. E. GABRlLovlTcH 479,338

INVERTER AND DISTORTER FOR SECRET COMMUNICATIONS Filed Jan. 13, 1945 2 sheets-sheet 2 EQUAL AMPLIFI PHASE SHIFTER DEMoDuLAToR Patented Aug. 16, 1949 UNITED STATES PATENT @FFHQE INVERTER AND DISTORTER FOR SECRET I COMMUNICATIONS claims. l

My invention relates to inverters and distorters for secret radio or telephonie communications and has particular reference to devices for distorting speech by inversion for rendering the speech unintelligible until it is reinverted at a. receiving point.

As is known in the art, inversion consists in modulating the speech frequency band to be transmitted, on an oscillation whose frequency is beyond the upper limit of the particular speech band but very vclose to this upper limit, and in filtering out the upper side band of the thus formed modulated Wave so as to maintain only the lower side band. It Vcan be readily realized that-this lower side band is located in the same frequency range as the initial speech band, but that all the frequencies of which this initial band is constituted, are inverted, so that in any pair of such frequencies the upper frequency becomes the lower one and, inversely, the lower frequency becomes the upper one.

An inverted speech band is completely unintelligible. However, it can be easily reinverted, i. e. re-transformed into the original speech band. This result is obtained by modulating the inverted band on an oscillation having substantially the same frequency as that used previously for effecting the inversion of the initial speech band, and by suppressing (by means of low-pass filtering) the upper sideband of the thus produced modulated wave. The original speech band is thus restored.

It is obvious that any unauthorized and undesired listener can perform such a reinversion by using very simple and handy arrangements. He

can for example, convey the received inverted speech band to a balanced modulator which is at the same time acted upon by the output of a local oscillator whose frequency can be varied in a continuous way. The output of the balance modulator is conveyed through a low-pass filter whose characteristics can be changed in such a way that at any given moment this filter substantially weakens any oscillation whose frequency exceeds that of the local oscillator used.

The adjusting operation to be effected by the undesired listener consists in turning in a continuous way the dial controlling the frequency of the local oscillator (and at the same time, the tuning of the low-pass filter) until the received chaotic plurality of sounds is transformed into intelligible speech. This result can be in most cases obtained in a few seconds, so that inversion does not actually secure either secrecy or privacy. Y The main object of my invention is to provide means and methods to render the speech band 2 unintelligible by an independent factor, with or without the preliminary inversion of the speech band. I provide for this purpose an arrangement whereby the speech band, either inverted f or straight, is modulated by a sequence of harmonically related low frequencies thereby rendering the speech completely unintelligible. I further provide means to reestablish the intelligibility at the reception end by modulating the thus distorted speech band with the fundamental of the foregoing sequence of harmonically related frequencies, this fundamental being used in a proper phase relationship. Y

A further object of my invention is to provide means and methods to vary the phases of the modulated speech frequencies contained in a speech band in accordance with a predetermined pattern so that no demodulation can be obtained unless a demodulating oscillation is used Whose phase undergoes exactly the same variations.

Still another object of my invention is to provide means and method to Vary the phases of a particular speech band by modulating this band by a sequence of phase-varied oscillations, i. e. by a sequence of oscillation whose phase is made to vary periodically in accordance with a certain law.

Still another object of my invention is to provide means and method for periodically shifting frequencies of the speech bands by heterodyning them by frequency-varied oscillations whose vfrequency is made to vary, for instance, with the aid of frequency-varying means employed in the frequency modulation.

In a preferred (although purely illustrative) form the invention may be carried out in the following way: Y

Let it be assumed that the frequencies constituting the speech-band to be transmitted, range from 300 cycles to 3,000 cycles. This speech band is made to modulate a special heterodyning oscillation having a frequency located e. g. somewhere between 3,000 and 3,200 cycles. If the upper sideband of the thus produced modulated current is suppressed by low-pass filtering so that only the lower sideband is maintained, the initial speech band will be inverted. If the `frequencies forming the original speech band are designated by the symbol fh (wherein h is an index), the frequencies constituting the above mentioned lower sideband, will have the form e. g.

In order to prevent the unauthorized or unde- The receiver as shown in Fig. 2 comprises a, low pass filter |5 having its cut-01T approximately at 3500 cycles. This filter suppresses the sub-carrier while maintaining the whole signal band, which consists 0f the inverted speech modulated by the sequence of harmonically related low frequencies. This band is fed to a modulator IB wherein it is modulated by the phase-varied fundamental of the foregoing sequence. As was mentioned, this modulating operation transforms a distorting modulation produced by a group of harmonically related frequencies into a modulation by a single frequency which is equal to the sum of that of the fundamental and that of the highest harmonic employed. It has been assumed that the fundamental of the distorting sequence has a frequency of 50 cycles and that seven odd harmonics are used, so that the highest harmonic (the fifteenth) has a frequency of 750 cycles. This means that the output of the modulator I6 will be constituted by the inverted speech modulated by a single frequency of 800 cycles.

The output of the modulator I6 is fed to a demodulator and low pass filter I8, which provide a non-modulated inverted speech which is then reinverted in a conventional reinverter I9.

'I'he lower row (reference characters 20 to 3|) represents an arrangement designed to provide the phase varied modulating and demodulating oscillations to be applied to modulator I6 and to demodulator I1. 20 is a high pass filter which selects the special subcarrier on which is impressed the frequency standard. This standard is detected by a detector 2| and then fed to a clipper 22 to be transformed into a square wave.

The output of clipper 22 is divided into two channels. One of them contains a selector 23 selecting the oscillation used as fundamental of the distorting sequence. This oscillation is fed to the electronic switch 26 through two parallel channels, one of which contains a xed phase shifter 25, shifting the phase of the incoming fundamental by 90. The other channel is fed directly to the electronic switch 26. tronic switch is synchronized by an oscillation picked up by a selector 2li, which oscillation is identical to that provided in the transmitter by the selector I0 (see Figure 1). The output of the electronic switch is divided into two channels. One of them is directly fed to the modulator I6, the second is conveyed to a shape distorter 21 distorting the shape of a sine wave in such a way, that there appear all the harmonics of said wave (odd as well as even). A selector 28 picks up a determined harmonic, in our case, the sixteenth, which is conveyed to a clipper 29,. The output of the latter is fed to a W pass lter 30 and then to an equalizing amplifier 3| whose output acts on the demodulator Demodulator Il performs as follows:

If the inverted speech band is designated by the symbol Ei and if the symbol wo is defined by the equation wo=21r800 the plurality of oscillations acting on the input of demodulator l1 will have the following analytical f orm:

wherein ipv is the variable (jumping up and down) phase.

Selector 28 provides a phase varied oscillation of 800 cycles whose expression is:

SinMot-l-ww) This phase varied oscillation is transformed by the clipper 29 and the low pass filter 10 into the sequence:

Sin (wot-wv) -l-t Sin Swat-tw) sin Mount-hbo) -l-Jf sin Hanoi-11011)' The angle tbv can have only two values: zero degree and degrees.

During all the intervals of time when rlfv=0 the output of l. p. filter '30 has the form:

During al1 the intervals of time when glfv=90 the output of 30 is expressed as follows:

Ez' sin wat(sinwot+sin3w0t|-sinwot-l-sin'lwut) and the forms In both cases the final result will be `fili-E11 cos Scoot As 8w0=21r6400 the l. p. filter 3| having its cut olf at 3000` cycles will obviously eliminate the term E cos Swot so that only the inverted speech band Eiwill be maintained.

It will be understood that various features and principles of each of the embodiments of the invention above described or referred to may be utilized or substituted in the other embodiments.

While the invention has been described in de.- tail with respect to certain particular preferred examples, it will be understood by those skilled in the art after understanding the invention, that various changes and further modications may be made without departing from the spirit and scope of the invention, and it is intended therefore in the appended claims to cover all such changes and modifications.

What is claimed as new and desired to be secured by Letters Patent is: Y

1. A method for secret electrical transmission of intelligence consisting in modulating the original speech-band, by a sequence of harmonically related oscillations having different frequencies for distorting the-speech and to render the speech completely unintelligible, and in reestablishing intelligibility at the reception end, by modulating the distorted speechband by the fundamental of the said distorting sequence of harmonically related frequencies, taken in a particular phase relationship, whereby the sequence of several modulating frequencies is transformed into a single modulating frequency. J

2. A method of secret transmission of intelligence whichv consists in inverting speechband and in modulating the inverted speechband by a sequence of harmonically related oscillations having such frequencies as to distort the in- Vented-speech, and in re'establ-ishing intelligibilityby modulating the transmitted inverted andv disi-A torted speech by the fundamental of said distorting sequence of harmonically related frequencies used in a particular phase relationship, whereby the sequence of several modulating frequencies is transformed into a single modulating frequency, and reinverting the inverted speech.

3. A method of secret transmission of intelligence, consisting` in inverting a speechband and in modulating this inverted speechband by a sequence of harmonically related phase varied oscillations having different frequencies, and reestablishing intelligibility by modulating the transmitted inverted and distorted speech by the fundamental ofA said distorting sequence of harmonically related phase varied oscillations, the

fundamental being used in a proper phase relationship after having been subjected to the same phase variations as those of the distorting modulating oscillations, whereby the sequence of several modulating phase variedV frequencies is transformed into a single modulating frequency.

4. A method of secret transmission of intelligence, consisting in inverting a speechband by a phase varied inverting oscillation, modulating this inverted phase varied speechband by a sequence of harmonically related oscillations, and reestablishing Vintelligibility by modulating the inverted and distorted speechband with a corresponding chosen oscillation undergoing phase variations and by performing. the reinversion of the inverted speechband by a reinverting oscillation.

5. In a method of producing secret transmission of intelligence the steps which include producing a sequencel of harmonically related frequencies located in the voice spectrum below the upper limit of the speech band used; and modulating a speech band by said sequence thereby rendering the speech unintelligible.

6. A method of secret transmission of intelligence, consisting in producing a vsequence of harmonically related frequencies; modulating a speech band byv said sequence thereby rendering the speech unintelligible; and reestablishing the intelligibility of the speech by producing a. fundamental of the distorting sequence vof frequencies bymodulating with said fundamental the distorted speech, thereby obtaining a sing-le modulating frequency.

7.-. In a method of producing secret transmission` of intelligence the steps which include producing a sequence of harmonically related frequencies; equalizing the amplitudes of said frequencies; and modulating a speech band by the said sequence thereby rendering the speech unintelligible. Y

8. In a method of producingV secret transmission of intelligence the steps which include producing a sequence of harmonically related frequencies; equalizing the amplitudes of said frequencies; periodically shifting the phase of the sequence; and modulating a speech band by said sequence thereby rendering the speech uni-ntelligible. v

9-. In a method of producing secret transmission of intelligence the steps which include producing a sequence of harmonically related fre.- quencies; equalizing the amplitudes of said frequencies producing a synchronizing frequency; .periodically shifting the phase of the sequence bythe synchronizing frequency; and modulating a speech band by the sequence thereby rendering the speech unintelligible..

10. In a methodfof producing secret transmission ofintelligence the steps which include producing a sequence of harmonically related frequencies by'subrnitting a` low frequency oscillation to a shape distorting' operation for changing the shape of the Wave'in such a manner as to produce harmonics having relatively strong amplitudes; and modulating a speech band by said sequence.

11. .In a method of producing secret transmission of intelligence the steps which include producing a sequence of harmonically related` frequencies located Within the signal band in the voice. spectrum used below the upper end thereof; inverting the, speech band; and modulating the inverted speech band by said sequence.

12. A method of' secret transmission of intelligence, consisting Vin producing a sequence of, harmonically related frequenciesA producing a synchronizing oscillation; periodically shifting the phase of the sequence by the synchronizing oscillation; modulating the speech band by the sequence for rendering the speech unintelligible; transmitting the modulated speech band .With the synchronizing oscillation; producing a fundamental of the distorting sequence of fre.- quencies by displacing the phases of the harmonically related frequencies bythe transmitted synchronizing frequency; and modulating the distorted speech band by the fundamental frequency, thereby reestablishing the intelligibiiity ofthe speech.

13. An apparatus for secret communication, comprising means to transform speech into speech band frequencies; means to produce a sequence of harmonically related oscillations having different frequencies; means to modulate the speech band by the sequence; means to. transmit 'the modulated frequencies by radio; means to receive and to detect the distorted transmitted speech band;l means to produce a fundamental of the distorting sequence; means to transform the distorting modulation oi the speech band produced by the sequence of modulated frequencies into a single modul-ating frequency; and means to. modulate the distorted speech band by the fundamental of the distorting sequence thereby reestablishing the intelligibility` of the speech.

14. An apparatus for secret communication, comprising means to transform speech into speech band. frequencies; means to produce a sequence of harmonically related oscillations having different frequencies; means to change the` phase relationship of the sequence; means to modulate the speech band byy the sequence; means to transmit the modulated frequencies by radio; means to receive and to detect the transmitted distortedspeech band; means to produce a fundamental of the distorti-ng sequence in the same phase relationship for transforming the fundamental of the distorting. sequence into a single modulating frequency; and means to modulate the distorted frequencies thereby reestablishing the intelligibility of the speech..

15. An apparatus for secret communicatinn, comprising means to transform speech into speech band frequencies; means to produce a sequence of harmonically related oscillations having different frequencies; means to;v produce a synchronizing frequency; means to chan-ge the phase relationship of the sequence bythe synchronizing frequency; means to modulate the speech band by the sequence; means to transmit. and to, detect the distorted speech band;

9 l0 means to produce a fundamental of the distort- REFERENCES CITED ing sequence in the phase relationship as controlled by the transmitted synchronizing fremy; ftswlrferens are of record m the quency for transforming the fundamental of the p distorting sequence into a single modulating fre- 5 UNITED STATES PATENTS quency; and means to modulate the distorting frequencies thereby reestablishing the intelligi- Iukels Chix al Jarj'ztels bum "f the speech' 2,169,357 Hansen Aug. 15, 1939 LEONIDE E. GABRILOVITCH. 2,395,432 Sprague Feb. 26, 1946 

